At present, the passive infrared sensor (PIR), which is used to detect the human being, adopts the lens array made of high-density polyethylene (HDPE) to focus the infrared ray emitted by the human being. In general, a lens can be categorized into a traditional Fresnel one or a general spherical one. The configuration of the array depends on the type of the lens mentioned above. The array adopting the Fresnel lenses is usually a flat and rectangular sheet body 10, as shown in FIG. 1 and FIG. 2, and one smooth face thereof is used as the light-exiting face 12, and the other face has many carved concentric circles 14 and functions as the light-entering face 16. Each of the carved concentric circles 14 can be regarded as a mini prism, and after being refracted by the mini prism, light is deflected to the focus, and thereby, lights are concentrated. The other type of lens array 20, as shown in FIG. 3, is composed of general spherical lenses 22.
However, there are some problems existing in the practical application of the abovementioned lenses:                1. The traditional Fresnel lens utilizes the principle of prism to focus light, and whether lights can be concentrated on one point depends on the density of the carved concentric circles; if the density too low, the focusing will lack sufficient accuracy; if the density too high, the incident light will not be focused but be diffracted. U.S. Pat. No. 4,787,722 discloses a Fresnel lens, wherein the light-entering face has multiple carved grooves, and the widths of the carved grooves gradually shrinks from the center to the edge to enable the depths of the carved grooves to be uniform, which can overcome the problem resulting from the past design that the grooves of the Fresnel lens have identical widths nevertheless without identical depths, which results in that the Fresnel lens is unable to be thinned, owing to that the thickness thereof is limited by the depths of the carved grooves. However, the Fresnel lens with array of the abovementioned grooves is to be bent into a columnar shape, which results in that the primary optical axis won't be perpendicular to the lens, which further induces comatic aberration and energy loss.        2. In contrast to flatness, thinness and lightness of the Fresnel lens, the spherical lens has two spherical surfaces, and the lenses used to detect objects in various directions and at various distances are assembled into a lens array. One surface of the spherical lens can be a spherical surface having a given radius of curvature; thus, the spherical lens has a ball-like appearance, as shown in FIG. 3. Owing to the ball-like appearance, the conventional spherical lens array has the advantage that all the primary optical axes are perpendicular to the lenses when detecting the infrared ray emitted from human body, which results in a superior focusing effect. In contrast to the thinness of the Fresnel lens, when the aperture of the lens of spherical lens array is intended to increase, the thickness of the lens should be increased. However, increasing the thickness of the lens reduces the transmittance of infrared ray. Therefore, the lens aperture of such a kind of spherical lens array has to be maintained within a given range, and when the ambient temperature over 28° C., the detection distance will be reduced.        